1. Field of the Invention
The present invention relates to a method and device for detecting a pulse signal corresponding to marks from a recording medium on which the marks for generating clock signals are periodically recorded.
2. Related Background Art
Generally, reference pulse signals for generating a clock signal during reproduction are recorded on an information recording medium such as an optical disk or the like.
FIG. 1 is a block diagram showing an example configuration of an optical disk drive apparatus of a sample servo method. In the drawing, the reference numeral 13 is an optical disk used as an information recording medium, numeral 14 is a reproducing transducer, and reference numeral 15 denotes a binarizing circuit.
Signals read from optical disk 13 by reproducing transducer 14 are converted to binary signals by binarizing circuit 15. These binary coded signals are input into a pulse signal extracting circuit 16 and a code decoding circuit 18. From the pulse signal extracting circuit 16 into which binary signals are input, only clock generating reference pulse signals are output. Then, synchronous clock signals are output by way of a phase-locked loop (PLL) circuit 17 and a code decoding circuit 18. On the other hand, reproducing data signals are output from code decoding circuit 18 which also receives the binary signals.
FIG. 2 is a waveform diagram showing signals reproduced from a recording format of a general sample servo method. As indicated in this diagram, signals are composed of servo, address and data portions. "Pc" shown in the diagram represents a reference pulse signal for generating a clock signal and these signals appear at constant intervals "S". The pulse interval d from the pulse before the reference pulse signal Pc for generating clock signals to the point of the reference pulse signal Pc for generating synchronous clock is also constant. Further, this pulse interval is set for a specific interval to prevent the appearance of pulses at other positions. There may be cases when this pulse interval "d" is not made constant. Even in such cases, however, a provided pattern recognition function enables identification of the reference pulse signals Pc for generating the clock signal.
Illustrated in FIG. 3 is an example of a simplified pulse signal extracting circuit for the case of a constant pulse interval "d". FIG. 4 is a time chart showing operation of this extracting circuit.
With reference to FIG. 3, reference numeral 19 represents a retriggerable monostable multivibrator, reference numeral 20 represents a monostable multi-vibrator, and reference numeral 21 represents an AND circuit. When binary signal J is input into retriggerable monostable multivibrator 19, monostable multivibrator 20 outputs a gate signal with a predetermined delay as shown in I in FIG. 4. By inputting the gate signal I and binary signal J into the AND circuit 21, the pulse signal Pc is output from AND circuit 21 as indicated by K in FIG. 4, provided that such a pulse signal Pc exists while the signal I is at a high level. On the other hand, a pulse signal which appears on binary signal J during the period that the gate signal is not output (the period when the signal I is at a low level) is not output from the AND circuit 21. In this manner, only a reference pulse signal for generating a clock signal can be extracted from other signals.
However, when the recording medium involves any defects or dust or foreign particles accumulated on the medium, the pulse signal extracting circuits may fail to output clock generating reference pulse signals. In such a case, there would be no reference for generating the gate signal. To solve this problem, a method for instance, of using other circuits (not shown in the figures) may be employed to generate a pseudo-pulse signal for input into the PLL circuit. A gate signal can be generated by using these pseudo-pulse signals as the reference to extract the pulse signal after missing a reference signal. On the other hand, if output timing of pseudo-signals may extensively deviate from the timing of the pulse signals to be actually detected, a deviation develops between the gate signal subsequent to a missing reference pulse and pulse signals read from the recording medium, resulting in a problem of continuous failure to detect pulse signals.